Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. I. Normal responses

Area of Science:

• Neuroscience
• Vestibular System Physiology
• Oculomotor Control

Background:

• The vestibuloocular reflex (VOR) stabilizes gaze during head movements.
• Understanding VOR dynamics at high frequencies and accelerations is critical for interpreting vestibular function and dysfunction.
• Previous models often simplified VOR responses, particularly under dynamic conditions.

Purpose of the Study:

• To investigate the horizontal angular vestibuloocular reflex (VOR) during high-frequency, high-acceleration rotations in squirrel monkeys.
• To characterize the frequency and velocity dependence of VOR gain and phase.
• To develop and validate a mathematical model accounting for observed VOR dynamics.

Main Methods:

• Recorded horizontal angular VOR in squirrel monkeys during steps of acceleration and sinusoidal rotations.
• Stimuli included high-acceleration steps (3,000 degrees /s²) and sinusoidal rotations (0.5-15 Hz) at varying peak velocities.
• Analyzed response trajectories using polynomial regression and developed a mathematical model incorporating linear and nonlinear pathways.

Main Results:

• VOR latency was 7.3 ± 1.5 ms for acceleration steps.
• VOR gain during acceleration was higher than at plateau velocity.
• At 20 degrees /s, VOR gain was constant across frequencies (0.5-15 Hz), but increased with velocity at frequencies ≥ 4 Hz.
• A cubic polynomial better represented acceleration step responses than a linear fit.
• Eye movements were negligible in labyrinthectomized animals, confirming vestibular origin.

Conclusions:

• The VOR exhibits frequency- and velocity-dependent nonlinearities, particularly at higher frequencies and velocities.
• A mathematical model incorporating linear and nonlinear pathways, including a cubic velocity term, accurately describes experimental findings.
• This model provides a framework for interpreting VOR responses and understanding vestibular deficits.